gpio: f7188x: use the new open drain callback

The F7188x chips supports setting the pins in open drain mode.
Activate the new .set_single_ended() callback.

Cc: Peter Hung <hpeter@gmail.com>
Cc: Andreas Bofjall <andreas@gazonk.org>
Cc: Simon Guinot <simon.guinot@sequanux.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>